252 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME At the equator these waters move westward in the Atlantic and Pacific oceans. The westward current in the Atlantic Ocean at the equator divides, one branch flowing northward past the West Indies, through the Caribbean Sea and the Gulf of Mexico, and past Florida as the Gulf Stream. The equatorial stream in the Pacific divides near the islands of Australia, the northern branch becoming the Japan Current. It must not be thought, however, that the only cause of ocean currents is the wind. It is probable that differences in the densities of water due to differences in salt content and other factors also cause ocean currents. These ocean currents seldom average more than three or four miles per hour, but they tend to equalize the temperature of the globe by making the waters of the equatorial zones cooler and the waters of the polar regions warmer. These ocean currents, influencing the temperature of the winds that blow over them, make the climate more equable on the portions of the continents in line with these winds. The Japan Current divides on reaching the western coast of North America, the northward portion becoming the Alaska Current, and the southward portion becoming the California Current. Arid Regions Are Found on the Leeward Side of Well-watered Mountain Ranges. The mountains along the Pacific coast in the United States produce conditions leading to the precipitation of much of the water from the winds that sweep up and over them ; and inasmuch as these mountains are transverse to the prevailing winds, the regions on the other side of the mountains receive very little rain. The Appalachians present no such effective barrier — they are not so high and are more or less parallel to the prevailing winds during much of the year. Climates Differ with Altitude. The temperature of the free air decreases on the average of 1° F. (though the rate of decrease varies widely from place to place and from time to time) for every three hundred feet of rise, so that the climate of the highest mountains resembles that of polar regions. The snow line represents the altitude at which snow is found most of the year. The snow line is higher with higher mountains and varies with the latitude, of course. The Inclination of the Earth's Axis to about the Sun Is the Cause of the Seasons. the Plane of the Earth's Orbit Figure 77 is based on observations that the axis of the earth is always inclined at an angle of 23 J^ degrees to the plane of the earth's
CLIMATE AND WEATHER 253 orbit. For this reason the north pole is indined toward the sun for half of the year, and the south pole is inclined toward the sun during the other half of the year. The two extreme positions occur on June 21 and December 22. The four seasons do not exist at the equator, and the days and nights there are of the same length. North of the equator the days become I II shorter from June 21 to December 22, while they become Fig. 76. I, the earth in June; II, the earth in December. (From the Yerkes Observatory, longer during the same period reprinted by permission of the Chicago University Press.) in the southern hemisphere. The diagram shows that the day is twenty-four hours long at the Arctic Circle on June 21, and the night is twenty-four hours long on December 22. The coldest period in the northern hemisphere at present is that time when the earth is closest to the sun, but every 13,000 years this condition is reversed because of precession. The distance from the earth to the sun varies from 91,500,000 miles to 94,500,000 miles, because the earth travels in an elliptical path. The seasons in the southern hemisphere are more extreme; and in the northern hemisphere it is warmer during the winter and cooler during the summer than it would otherwise be, because the earth is closest to the sun when it is winter in the north. Fall Equinox fZ^^.^ Day ano Night Equal 24HourDay Long Day l2-HourDay Spring Equinox Day and Nisht Equal Fig. 77. The seasons; a diagram of the earth in various positions in its orbit illustrating the cause of the seasons. The amount of heat received by the earth in the different latitudes is determined not only by the lengths of the days and nights but also by the angle at which the sun's rays strike the earth. Thus the winter is not as hot as the summer, just as the evening is not as hot as the noonday.